The present invention relates generally to a projection optical system, and more particularly to a catadioptric projection optical system that projects an object, such as a single crystal substrate and a glass plate for a liquid crystal display (“LCD”), using a mirror. The present invention is suitable, for example, an immersion exposure apparatus (an immersion lithography exposure system) for exposing the object through a liquid between the projection optical system and the object.
The photolithography technology for manufacturing fine semiconductor devices, such as semiconductor memory and logic circuits, has conventionally employed a reduction projection exposure apparatus that uses a projection optical system to project and transfer a circuit pattern of a reticle (or mask) onto a wafer, etc. A more highly integrated and finer semiconductor device (circuit pattern) requires a projection optical system for a better specification and performance. Generally, a shorter wavelength of the exposure light and a higher numerical aperture (“NA”) are effective to improve the resolution. Recently, an optical system with an NA of 1 or higher that utilizes an immersion optical system that fills a space with liquid between a final glass surface (in other words, the lens closet to the wafer) of the projection optical system and the wafer has been proposed, and a higher NA scheme is in progress.
For the exposure light with a short wavelength such as an ArF excimer laser (with a wavelength of approximately 193 nm) and a F2 laser (with a wavelength of approximately 157 nm) for higher resolution, lens materials are limited to quartz and calcium fluoride for reduced transmittance. An optical system that includes only lenses (refracting element) uses, generally, quartz and calcium fluoride for the exposure wavelength of 193 nm for instance. However, quartz and calcium fluoride possess similar dispersive powers, and have difficulties in correcting the chromatic aberration, especially, for the optical system that has a very higher NA as in the immersion optical system. Moreover, a lens diameter increases as the NA becomes higher, and causes the increased apparatus cost.
Then, various proposals that use a mirror (reflecting element) for an optical system have been made to solve the disadvantageous reduced transmittance, difficult corrections to the chromatic aberration and large aperture of the lens. For example, a catadioptric projection optical system that combines a mirror with a lens has been disclosed. See, for example, Japanese Patent Applications, Publication Nos. 2004-205698, 8-62502 and 2003-307679.
A projection optical system that includes a mirror for a shorter exposure wavelength and a higher NA needs to correct the chromatic aberration corrections, maintain a large enough imaging area on an image surface, and have latitude for a higher NA. Especially, when the NA of higher than about 1.1 dramatically increases an object-to-image distance (in other words, a distance between a reticle and a wafer), and an effective diameter of lens inevitably enlarging the optical system and increasing the apparatus cost.
The optical system disclosed in Japanese Patent Application, Publication No. 2004-205698 propose a twice-imaging catadioptric optical system for forming an intermediate image once. The optical system includes a first imaging optical system that has a reciprocating optical system (double-pass optical system) which includes concave mirror and forms an intermediate image of a first object (e.g., a reticle), and a second imaging optical system that forms the intermediate image onto a surface of a second object (e.g., a wafer). Moreover, the optical system arranges a first plane mirror near the intermediate image for deflecting an optical axis and light. The deflected optical axis from the first plane mirror is made approximately parallel to a reticle stage and is deflected once again by a second plane mirror, and images onto the second object. The optical system disclosed in Japanese Patent Application, Publication No. 2004-205698 has the above structure, and inevitably arrange the first object (e.g., a reticle), a lens, plane mirror and the deflected light close to one another. Therefore, the optical system creates a problem of interference between the first object (e.g., a reticle) surface or the reticle stage and the lens or the plane mirror or an insufficient space. The optical system in FIG. 5 of Japanese Patent Application, Publication No. 2004-205698 is an immersion optical system with an NA of 1.05. However, a maximum effective diameter is over φ300, and if NA achieves higher NA such as 1.2 or higher, the effective diameter is remarkably enlarged.
The optical system in FIGS. 7 and 9 of Japanese Patent Application, Publication No. 8-62502 is a catadioptric optical system with an NA from 0.45 to 0.5 for forming an image three times or an intermediate image twice. The optical system includes a first imaging optical system that forms a first intermediate image of a first object (e.g., a reticle), a second imaging optical system that includes a concave mirror and forms a second intermediate image from the first intermediate image, and a third imaging optical system that images the second intermediate image onto a second object (e.g., a wafer) surface. The second imaging optical system includes concave mirrors as a reciprocating optical system. In this optical system, the first object (e.g., a reticle) and the second object (e.g., a wafer) are not arranged in parallel. The imaging performance improves in a scanning exposure, and a stability performance can be maintained when the first and second objects are arranged in parallel to each other and perpendicular to the gravity. Therefore, it is undesirable for an exposure apparatus that has an optical system with a higher NA by the immersion etc. to arrange the first object (e.g., a reticle) and second object (e.g., a wafer) in not parallel. This optical system needs another plane mirror to arrange the first object (e.g., a reticle) and second object (e.g., a wafer) in parallel. In that case, as described in Japanese Patent Application, Publication No. 8-62502, if a mirror is arranged near the first intermediate image, the arrangement is the same as the optical system in FIGS. 4 and 6 of Japanese Patent Application, Publication No. 20003-307679 described later. If an optical axis of the lens (optical element) having a refractive power is not along the gravity direction, the imaging performance deteriorates by self-weight deformations and influences of retainer. Therefore, preferably, the optical element having a refractive power with the optical axis that is not along the gravity direction does not exist as much as possible. However, the optical system in FIGS. 7 and 9 of Japanese Patent Application, Publication No. 8-62502 includes plural optical element with the optical axis that is not along the gravity direction.
The optical system with an NA of almost 0.85 in FIGS. 4 and 6 of Japanese Patent Application, Publication No. 2003-307679 arranges a plane mirror (reflection block) near the first and second intermediate images, and aligns optical axes of the first and third imaging optical systems with each other. Thus, the first object (e.g., a reticle) and the second object (e.g., a wafer) are arranged in parallel. However, such an optical system becomes considerably large when the NA becomes 1 or higher as in the immersion optical system, especially, about 1.1 or higher. Since the first imaging optical system from the first object (e.g., a reticle) to the vicinity of the plane mirror and the third imaging optical system from the vicinity of the plane mirror to the second object (e.g., a wafer) are arranged on a straight line of the optical axis, a sum of the object-to-image distance of the first imaging optical system and the object-to-image distance of the third imaging optical system becomes the the object-to-image distance (the distance between the reticle and the wafer) of the entire optical system. Due to the strong refractive power of each lens necessary to maintain the size of the optical system with a higher NA, the aberration correction becomes difficult. Moreover, because of a small reduction magnification in the first imaging optical system, the first intermediate image enlarges the NA of the first intermediate image for an object-side NA in the first object (e.g., a reticle) by the reduction magnification. As a result, both an incident angle range and the maximum incident angle upon the plane mirror increase, and a higher NA scheme as in the immersion etc. encounters a serious problem. In other words, both the incident angle range and the maximum incident angle upon the plane mirror considerably increase due to the NA of 1 or higher, and the imaging performance inevitably deteriorates by the influence of a deteriorated property of the plane mirror etc. Because the plane mirror is also arranged near the second intermediate image, this is true of the second intermediate image. Moreover, as the above-mentioned, optical axes of the concave mirror and the negative lens in the second imaging optical system are perpendicular to the gravity direction, and it is not desirable structure to improve a final performance to a limit.